1. Field of the Invention
This invention relates to the field of computer tomography and more specifically to the use of computer tomography with the manufacturing of metal. In particular, a computer tomography system is disclosed for measuring a variety of dimensions associated with a metal sample.
2. Background Art
As part of the steel manufacturing process, a variety of methods have been used to determine various dimensions (including the height, width, thickness and surface flatness) of the finished product. These methods typically rely upon one of two technologies: optics and radiation.
Optics (and optoelectronics) have been used in various industries to determine object dimensions. For example, in a system disclosed by Wiese, ("Improving Quality with Optoelectronic Measurement" Tooling and Production, Vol. 49, pp 72-77, April 1983) various optoelectronic sensing techniques are described for use in metal working and manufacturing. One of the techniques measures the flatness of hot steel strips by triangulation of reflected light rays. Another technique measures the thickness of strip steel by separately triangulating the light rays reflected from the top and bottom surface of the steel sheet.
A further example of an optical system is disclosed by Pirlet ("A Noncontact System for Measuring Hot Strip Flatness", Iron and Steel Engineer, Vol. No. 7, pp 45-50, 1983). In this reference, an optical system is described which measures the flatness of sheet metal in a hot strip finishing mill. The system reflects light supplied by multiple helium-neon lasers from the steel sheet onto corresponding photodiode arrays to triangulate the position of the surface of the steel sheet along three to five lines that run the length of the sheet.
While optics systems have typically achieved good results in a variety of applications, optic systems are not well suited for the steel manufacturing environment. This environment creates a large amount of dust and dirt which can easily interfere with optical sensing. Thus, many steel manufacturers have attempted to obtain measurements of their finished product by using beams of high energy radiation and radiation detectors to determine product dimensions.
One prior art system disclosed in Smith (U.S. Pat. No. 4,047,036) relates to a thickness gauge in which two partially overlapping antiphasal fan beams of radiation are directed towards an object to determine the thickness of the object. An array of scintillation detectors below the object detects the radiation from the two fan beams.
Another prior art system (Adams "On-Line Measurement of Hot Strip Profile", Steel Times, Vol. 207, No. 2, pp 135-136, February 1979) also uses radiation to determine the thickness of the object. This system uses a fixed C-frame center line gauge and a traversing gauge to generate a zigzag thickness profile. In the time it takes to completely traverse the width of the object several hundred feet of the object may have passed the gauge. Thus, a considerable portion of the object is not examined.
A further prior art system disclosed by Kanamori ("Application of Gamma Ray Computed Tomography to Non-Destructive Testing" Nuclear Engineering and Design, Vol. 94, No.3, pp 421-426, 1986) uses a gamma ray computer tomography (CT) scanner which employs a fan beam of gamma rays that is extendable up to 20 degrees, and a 20 channel gamma ray scintillation detector that has a counting rate of up to 10.sup.5 per second. This system is limited to the inspection of low density materials such as plastics, ceramics and aluminum.
In a system disclosed by Matsuura ("Industrial. X-ray CT Scanner", WCNDT, pp 693-700, 1985) two industrial CT scanners are used to determine the dimensions of the object. One has a 140 KV X-ray source for use with ceramics, plastics, rubber and aluminum products and one with a 420 KV X-ray source for use with zirconia, ceramics, and steel. Both of the scanners employ a fixed source and a fixed array of 512 high pressure xenon detectors. The object to be imaged is placed on a rotating table between the source and the detector array. Thus, the system is not suitable for sheet steel production, in which the finished product must be continually examined as it is manufactured.
In a further Computer Tomography system disclosed by Hoffman et al., (U.S. Patent No. 4,951,222), which is hereby incorporated by reference for its teachings on tomography in the steel industry, a fan-shaped beam of radiation is generated and divided into fan ray elements. These fan ray elements are directed to a planar section of a structural steel object, such as an I-beam, and detected by a set of detectors. Each detector produces a signal representative of the intensity of the radiation of a detected fan ray element and selected coordinates defining a cross-sectional image of the object are determined from the intensity signals of the detectors. Because the source rotates about the object as it moves, the object is subject to a helical scan of its dimensions. Thus, areas of the object may not be examined.